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Just read the documentation:

src – input array (single-channel, 8-bit or 32-bit floating point).
dst – output array of the same size and type as src.
thresh – threshold value.
maxval – maximum value to use with the THRESH_BINARY and THRESH_BINARY_INV thresholding types.
type – thresholding type (see the details below).

Obviously maxval is only used with thresh_binary and its inverted variant. So just put in some random number for maxval. It will be ignored anyways.